the presence of such leaks are not always readily detected. It is also possible by leakage from different couples to the ground to obtain very erratic and erroneous readings when the common return is employed. Base metal couples are frequently constructed with the hot junction welded to the end of the iron protecting tube in order to reduce thermal lag. Even when this welded junction is not made the hot junction usually touches the protecting tube and is in good electrical contact with it especially since at high temperatures, insulation resistance becomes very low. Suppose that the iron tubes of two chromel-alumel couples are grounded to the iron casing of a furnace. The two hot junctions are thus connected to each other by a circuit of iron. The electrical circuit is represented by Fig. 10. On the indicator we have a chromel-alumel couple one leg of which is shunted by an alumel-iron-alumel differential couple. If the temperatures of the hot junctions of the couples are the same, this differential couple produces no effect. It will, however, alter the reading of the indicator whenever the two temperatures differ. Both the chromel-alumel couples will accordingly give erroneous results. If individual returns are used the iron circuit produces no effect. When grounds occur further back from the hot junction, for example between the common return and the other lead wire of a single couple, all couples on the common return have in addition to their own electromotive force an impressed IR drop due to the current flowing in the shunted couple. This may cause a large error in the reading of every couple on the line. Installations employing a common return are extensively used in the industries. It is a dangerous practice and one which should be avoided as far as possible.
Wiring Diagrams of Thermocouple Installations.—Figure 11 illustrates a simple thermoelectric installation for a rare-metal couple. The couple is properly protected by a porcelain or quartz tube and if necessary by an outer tube of iron, chromel, fireclay, etc. From the head of the couple compensating lead wires are carried to the bottom of a pipe driven 10 ft. under ground. From the bottom of the pipe copper lead wires are carried to the indicator.
Figure 12 illustrates a multiple installation for five thermocouples. In this case a common return is employed although as stated above the use of an individual return is preferred. By use of the common return for this installation four lengths of copper wire from the couples to the recorder have been saved, and the commutating switch is simpler. The indicator for the operator of the furnaces and the recorder for the superintendent's office are mounted in parallel. The indicator or recorder may be connected to any couple desired by setting the commutating switch illustrated in the lower halves of the cases of the instruments. Such an installation can be employed only when the instruments have a high resistance. The recorder and indicator when connected to the same couple at the same time act as a shunt on each other and this tends to make both instruments read low, whereas if the two instruments are calibrated to read correctly in parallel they will both read high when connected to different couples. An example illustrates this point. Suppose the line and couple resistance for each circuit is 3 ohms and the resistances of the recorder and indicator are 500 ohms
+ H- -t 4
*o3 o 1 1 1 s 1 r .*" 5
S 1 1 J3 O a 0
v/ Iron v/ v/ ^— ^ \/
FIG. 10.—Typical pyrometer circuits.